This is a pure bismuth crystal. The heaviest element that is not radioactive (ok technically it is but it’s half life is like 9 orders of magnitude older than the universe so it really doesn’t count.) Probably my favourite crystal structure, even if you forget the colour. Surprisingly, bismuth is also super-not-toxic. You can actually eat the stuff and it’s often in indigestion remedies. Fascinating element, all round.
This image of an amazing Amethyst geode is from a Squidoo article called "Grow Your Own Crystal Geodes" by Dr. Anne Helmenstine. Although this geode is natural rather than homegrown, I still think it sounds really fun to grow your own geodes!